Liquid crystal displays have been widely applied as display devices for personal computers, portable terminals and the like. Further improvement in luminance and energy-saving properties has been sought for panel-form illuminating system as the back lighting of display devices.
One embodiment of a conventional panel-form illuminating system is explained by referring to FIG. 34 (see Laid-open (Kokai) Japanese Patent Application No. Hei 5-127159.).
FIG. 34 is a cross-sectional view of the conventional panel-form illuminating system. As shown in the figure, the conventional panel-form illuminating system consists of a flat-panel photoconductor 131, a linear light source 134 parallel to the side of photoconductor 131, a reflector 135 covering linear light source 134, a light diffusing material 136 applied on the back side of photoconductor 131 in a spot pattern, a reflective sheet 133 applied below the back side of photoconductor 131, a light diffusing sheet 132 applied above the front side of photoconductor 131, and a prism sheet 137 applied above light diffusing sheet 132.
In this conventional panel-form illuminating system, light is irradiated from the side of photoconductor 131 by linear light source 134. The light transmitting inside photoconductor 131 by total reflection is diffused by light diffusing material 136 on the back surface of photoconductor 131. Due to light diffusion, the conditions of total reflection deteriorate, and the light emits to the outside of photoconductor 131. The light emitting from photoconductor 131 is reflected by reflective sheet 133 formed on the back surface of photoconductor 131, thus emitting the light from the front surface of photoconductor 131. Since the light emits in an inclined direction from the front surface of photoconductor 131, the light is refracted to the front surface direction by prism sheet 137. Due to the application of light diffusing sheet 132 above the front surface of photoconductor 131, the pattern of light diffusing material 136 on the back surface of photoconductor 131 cannot be observed from the front surface of photoconductor 131.
In the above-described conventional panel-form illuminating system, however, multiple reflection occurs between prism sheet 137 and reflective sheet 133. Thus, light absorption at light diffusing material 136, light diffusing sheet 132 and reflective sheet 133 becomes large, so that optical efficiency as a panel-form illuminating system declines. Furthermore, with the application of a plurality of sheets (reflective sheet 133, light diffusing sheet 132 and prism sheet 137), each sheet is likely to deviate from the correct position and dust can enter between sheets, thereby further reducing quality.